Mutations in the SCN1A voltage-gated sodium channel (VGSC) are responsible for a growing number of disorders, including Dravet syndrome (DS) and genetic epilepsy with febrile seizures plus (GEFS+). DS is a catastrophic early-life encephalopathy associated with prolonged and recurrent early-life febrile seizures (FSs), treatment-resistant afebrile epilepsy, ataxia, and intellectual disability. GEFS+ is an inherited disorder characterized by FSs that persist beyond the age of six and the development of a wide range of adult epilepsy subtypes. To date, research has focused mainly on the seizure component of these disorders; however, we now recognize that seizure onset is often followed by cognitive stagnation/decline, hyperactivity, and the development of a number of behavioral comorbidities, including autistic and psychotic traits. Although seizure frequency does tend to decline during adulthood, these comorbidities persist and are a major challenge in the clinical management of this patient population. Currently, the cognitive and behavioral outcomes associated with SCN1A dysfunction are poorly characterized, and the factors that influence the severity of these deficits unknown. Mitigating the impact of these comorbidities on quality-of-life outcomes will require broader research efforts to better characterize these cognitive and behavioral deficits, as well as identify factors that influence their severity. Towards this end, te proposed experiments in this R21 application will use Scn1a mouse models developed in our laboratory to address several fundamental gaps in our knowledge. Specifically, we will 1) better define the cognitive and behavioral deficits that result from altered SCN1A function, 2) determine whether prolonged early-life FSs, which are a common clinical feature of SCN1A-derived epilepsies, are likely to contribute to the worsening of cognitive and behavioral outcomes, and 3) determine whether the emergence of cognitive and behavioral abnormalities requires altered SCN1A function during the early period of postnatal brain development or is an invariant (age-independent) outcome of altered SCN1A function. These experiments are both innovative and clinically relevant, and will stimulate much-needed translational research.